太湖沉积对流域极端降水和洪水响应的研究

Study on Lacustrine Sediments Responsing to Climatic Precipitation and Flood Discharge in Lake Taihu Catchment,China

认识极端洪水特征和周期,急需建立长期洪水记录和序列。过去150 a来长江下游极端降水引发了多次太湖特大洪水,太湖湖泊沉积提供了长于观测资料的洪水记录。本文利用太湖中心开阔水域的近现代沉积,采用210Pb和137Cs测年法和粒度、磁学特征分析,与区域夏季降水和长江下游洪峰流量进行相关分析,恢复了太湖流域150 a来的历史洪水事件。根据长江下游极端夏季降水(Pjja≥90%百分位)和极端径流(Q≥90%百分位)以及历史文献记载,太湖流域自1840年以来约有24次特大洪水年,而湖泊沉积物砂级粒径与低频磁化率等特征能捕获与之对应的中洪水事件15次。这表明湖泊沉积记录能较好地反演过去洪水变化,为利用沉积记录认识百年极端洪水长周期变化和特征提供了沉积学、磁学等方面的科学依据。

It is necessary to know long-term flood records and the sequence for understanding the features and periods of extreme flood, while lacustrine sediments from Taihu Lake has provided the records much longer than the observa- tions. This paper has reconstructed historical flood events during the past 150 years in Taihu Lake catchment, by sedi- mentary records from TXS core in the centre of the open water. Dating of the 210Pb and 137Cs and characteristics of the grain size and magnetic susceptibility were analyzed, and sedimentary flood index were configureated which have been correlated with regional summer precipitation and flood discharge in the lower reaches of the Yangtze River. Results showed that there were about 24 events of the extreme large floods when we used cutoff the 90＇h pereentiges for the dis- charge （Q≥90th%） and summer precipitation （Pjja ≥90th% ） from historical documents since 1840AD. Compari- ably, flood index from grain size of sedimentary sand （ 〉 64 μm） and low-frequency magnetic susceptibility can catch 15 flood events during the past 150 years, 62.5% corespending with the historical records. This result supported that lacustrine sediment records can well reconstruct the paleoflood change, and provid scientific basis of sedimentology and magnetic susceptibility to recognize centenary timescale extreme floods in features and periods.